T. Gietzelt, T. Wunsch, V. Toth, T. Traut, W. Fürbeth
S. Lück (Hrsg.), Compact Heat Exchangers, 2nd revised ed., PP PUBLICO Publications, Essen, ISBN 3-934736-42-4 (2018) S. 180-204
Micro process apparatuses show large surface-to-volume ratios. These are mostly achieved by microstructuring of thin sheet material, which is stacked and joined subse-quently by diffusion bonding because this method is also suitable for joining internal cross sections. Due to the small cross sections of microchannels, the resulting forces are low even for high pressures of up to several hundred bars. Due to miniaturization, the material’s microstructure and the mechanical microstruc-tures are of the same order of magnitude. Hence, corrosion resistance is essential for the structures’ lifetime. Due to the level and duration of high temperatures during diffusion bonding and due to low cooling rates achievable in vacuum, the corrosion properties of the materials are decreased. Grain growth occurs, and precipitations are formed at the grain boundaries. Due to the so-called sensitization, intergranular corrosion may occur. If the wall that separates the reaction and cooling passage is formed by a single grain only, failure
may ensue. Corrosion tests e.g., ASTM G28-A, are not helpful for micro apparatuses with wall thicknesses of some hundred micrometers because:
• extrapolation from a standardized corrosion medium to different compositions is inadequate,
• testing duration is too short for obtaining results in practice, and
• the criterion for a depth of 50 μm of intergranular corrosion attacks in conjunction with short test durations gives no reasonable prediction for micro process apparatuses.
In an AiF research project, four different Ni-base alloys were tested. Samples were tested in 70 and 95-97% sulfuric acid at 85 and 100°C for 1000 h, respectively. Despite minor improvements, no alloy fulfilled the requirements to be addressed for micro process devices. Hence, also different coating technologies were included. A commercially available tantalum coating made by a CVD process was found to be the solution of choice. Ta-coatings exhibit superior adhesion. No defects could be found due to their thicknesses of more than 10 μm. The thicknesses are uniform along microchannels with small cross sections and large lengths. By this, expensive and difficultto-provide semi-finished products of Ni-base alloys can be substituted by austenitic stainless steels.
Additional cost benefits arise from microstructuring of austenitic stainless steels by means of chemical etching. Mechanical microstructuring of tough materials, however, is expensive and special know-how is required. However, new issues arise, which should be investigated in detail before this approach is used in practice:
• Which influence has the surface morphology of the tantalum coating regarding pressure loss and fouling in microchannels e. g., due to side reactions?